In the above image forming devices, various organic photosensitive materials having a sensitivity at a wavelength range of a light source used in said devices have been used. Recently, the organic photosensitive material has widely been used because of easy production in comparison with a conventional inorganic photosensitive material, various selective photosensitive materials (e.g. electric charge transferring material, electric charge generating material, binding resin, etc.) and high functional design freedom.
Examples of the organic photosensitive material include a single-layer type photosensitive material wherein an electric charge transferring material and an electric charge generating material are dispersed in the same photosensitive layer, and a multi-layer type photosensitive material comprising an electric charge generating layer containing an electric charge generating material and an electric charge transferring layer containing an electric charge transferring material, which are mutually laminated.
Japanese Patent Kokai Publication No. 7-244389 discloses a multi-layer type electrophotosensitive material containing a stilbene derivative represented by the general formula (6): ##STR2## (wherein R.sup.A, R.sup.B, R.sup.C and R.sup.D represent a hydrogen atom, an alkyl group which may have a substituent, an aryl or alkyl group which may have a substituent, or an alkoxy group; and R.sup.E and R.sup.F represents a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group or an aryl group) as a hole transferring material in a sensitive layer.
In the case of the multi-layer type photosensitive material, it is possible to select a positive or negative charging type according to the laminating order of an electric charge generating layer and an electric charge transferring layer as well as kind of an electric charge transferring material to be contained in the electric charge transferring layer. However, since a film thickness of the electric charge generating layer is smaller than that of the electric charge transferring layer, the strength of the surface of the photosensitive material becomes insufficient when the electric charge transferring layer is formed on a conductive substrate and then the electric charge generating layer is formed thereon. Therefore, it becomes necessary to provide a protective on an outer-most layer, thereby to cause deterioration of optical characteristics of the photosensitive material. Therefore, there has hitherto been used a construction, wherein the electric charge transferring layer is formed on the outer-most layer, very often.
Regarding the electrophotosensitive material disclosed in the above publication, as described above, the above stilbene derivative (6) is used as the electric charge transferring material to be contained in the electric charge transferring material in the construction wherein the electric charge transferring layer is provided on the outer-most layer.
However, since this stilbene derivative (6) is a hole transferring material, the charging type of the photosensitive material becomes a negative charging type. As a result, it becomes necessary to use negative-polar corona discharging, which causes generation of a large amount of ozone, when the photosensitive material is charged, thereby causing such a problem that deterioration of the photosensitive material is promoted by ozone generated on charging and a bad influence is exerted on the environment.
There was also a problem that the photosensitivity of the resulting multi-layer type photosensitive material has never reached a level suited for practical use because charge transfer of the above stilbene derivative (6) is not sufficiently large.